Wire grid polarizer

ABSTRACT

A wire grid polarizer (300) for polarizing an incident light beam (130) comprises a substrate having a first surface. A grid or array of parallel, elongated, composite wires (310) is disposed on the first surface (307), and each of the adjacent wires are spaced apart on a grid period less than a wavelength of incident light. Each of the wires comprises an intra-wire substructure (315) of alternating elongated metal (330a-i) wires and elongated dielectric layers (350a-i).

CROSS REFERENCE TO RELATED APPLICATIONS

Reference is made to commonly-assigned U.S. patent application Ser. No.09/799,281, now U.S. Pat. No. 6,532,111 B2 filed Mar. 5, 2001, entitledWIRE GRID POLARIZER, by Kurtz et al. and U.S. patent application Ser.No. 09/977,544, filed Oct. 15, 2001, entitled DOUBLE SIDED WIRE GRIDPOLARIZER, by Silverstein et al., the disclosures of which areincorporated herein.

FIELD OF THE INVENTION

The present invention relates to wire grid polarizers in general and inparticular to multilayer wire grid polarizers and beamsplitters for thevisible spectrum.

BACKGROUND OF THE INVENTION

The use of an array of parallel conducting wires to polarize radio wavesdates back more than 110 years. Wire grids, generally in the form of anarray of thin parallel conductors supported by a transparent substrate,have also been used as polarizers for the infrared portion of theelectromagnetic spectrum.

The key factor that determines the performance of a wire grid polarizeris the relationship between the center-to-center spacing, sometimesreferred to as period or pitch, of the parallel grid elements and thewavelength of the incident light. If the grid spacing or period is longcompared to the wavelength, the grid functions as a diffraction grating,rather than as a polarizer, and diffracts both polarizations, notnecessarily with equal efficiency, according to well-known principles.However, when the grid spacing (p) is much shorter than the wavelength,the grid functions as a polarizer that reflects electromagneticradiation polarized parallel (“s” polarization) to the grid, and.transmits radiation of the orthogonal polarization (“p” polarization).

The transition region, where the grid period is in the range of roughlyone-half of the wavelength to twice the wavelength, is characterized byabrupt changes in the transmission and reflection characteristics of thegrid. In particular, an abrupt increase in reflectivity, andcorresponding decrease in transmission, for light polarized orthogonalto the grid elements will occur at one or more specific wavelengths atany given angle of incidence. These effects were first reported by Woodin 1902, and are often referred to as “Wood's Anomalies.” Subsequently,in 1907, Rayleigh analyzed Wood's data and had the insight that theanomalies occur at combinations of wavelength and angle where a higherdiffraction order emerges. Raleigh developed the following equation topredict the location of the anomalies, which are also commonly referredto in the literature as “Rayleigh Resonances.”

λ=ε(n+− sin θ)/k  (1)

where epsilon (ε) is the grating period; n is the refractive index ofthe medium surrounding the grating; k is an integer corresponding to theorder of the diffracted term that is emerging; and lambda and theta arethe wavelength and incidence angle (both measured in air) where theresonance occurs.

For gratings formed on one side of a dielectric substrate, n in theabove equation may be equal to either 1, or to the refractive index ofthe substrate material. Note that the longest wavelength at which aresonance occurs is given by the following formula:

λ=ε(n+ sin θ)  (2)

where n is set to be the refractive index of the substrate.

The effect of the angular dependence is to shift the transmission regionto larger wavelengths as the angle increases. This is important when thepolarizer is intended for use as a polarizing beamsplitter or polarizingturning mirror.

In general, a wire grid polarizer will reflect light with its electricfield vector parallel (“s” polarization) to the wires of the grid, andtransmit light with its electric field vector perpendicular (“p”polarization) to the wires of the grid, but the plane of incidence mayor may not be perpendicular to the wires of the grid as discussed here.Ideally, the wire grid polarizer will function as a perfect mirror forone polarization of light, such as the S polarized light, and will beperfectly transparent for the other polarization, such as the Ppolarized light. In practice, however, even the most reflective metalsused as mirrors absorb some fraction of the incident light and reflectonly 90% to 95%, and plain glass does not transmit 100% of the incidentlight due to surface reflections. The performance of ire gridpolarizers, and indeed other polarization devices, is mostlycharacterized by the contrast ratio, or extinction ratio, as measuredover the range of wavelengths and incidence angles of interest. For awire grid polarizer or polarization beamsplitter, the contrast ratiosfor the transmitted beam (Tp/Ts) and the reflected beam (Rs/Rp) may bothbe of interest.

Historically, wire grid polarizers were developed for use in theinfrared, but were unavailable for visible wavelengths. Primarily, thisis because processing technologies were incapable of producing smallenough sub-wavelength structures for effective operation in the visiblespectrum. Nominally, the grid spacing or pitch (p) should be less than˜λ/5 for effective operation (for p ˜0.10-0.13 μm for visiblewavelengths), while even finer pitch structures (p˜λ/10 for example) canprovide further improvements to device contrast. However, with recentadvances in processing technologies, including 0.13 μm extreme UVphotolithography and interference lithography, visible wavelength wiregrid structures have become feasible. Although there are severalexamples of visible wavelength wire grid polarizers devices known in theart, these devices do not provide the very high extinction ratios(>1,000:1) across broadband visible spectra needed for demandingapplications, such as for digital cinema projection.

An interesting wire grid polarizer is described by Garvin et al. in U.S.Pat. No. 4,289,381, in which two or more wire grids residing on a singlesubstrate are separated by a dielectric interlayer. Each of the wiregrids are deposited separately, and the wires are thick enough (100-1000nm) to be opaque to incident light. The wire grids effectively multiply,such that while any single wire grid may only provide 500:1 polarizationcontrast, in combination a pair of grids may provide 250,000:1. Thisdevice is described relative to usage in the infrared spectrum (2-100μm), although presumably the concepts are extendable to visiblewavelengths. However, as this device employs two or more wire grids inseries, the additional contrast ratio is exchanged for reducedtransmission efficiency and angular acceptance. Furthermore, the deviceis not designed for high quality extinction for the reflected beam,which places some limits on its value as a polarization beamsplitter.

A wire grid polarization beamsplitter for the visible wavelength rangeis described by Hegg et al. in U.S. Pat. No. 5,383,053, in which themetal wires (with pitch p<<λ and ˜150 nm features) are deposited on topof metal grid lines, each of which are deposited onto a glass or plasticsubstrate. While this device is designed to cover much of the visiblespectrum (0.45-0.65 μm), the anticipated polarization performance israther modest, delivering an overall contrast ratio of only 6.3:1.

Tamada et al, in U.S. Pat. No. 5,748,368, describes a wire gridpolarizer for the near infrared spectrum (0.8-0.95 μm) in which thestructure of the wires is shaped in order to enhance performance. Inthis case, operation in near infrared spectrum is achieved with a wirestructure with a long grid spacing (λ/2<p<λ) rather than the nominalsmall grid spacing (p˜λ/5) by exploiting one of the resonances in thetransition region between the wire grid polarizer and the diffractiongrating. The wires, each ˜140 nm thick, are deposited on a glasssubstrate in an assembly with wedge plates. In particular, the deviceuses a combination of trapezoidal wire shaping, index matching betweenthe substrate and a wedge plate, and incidence angle adjustment to tunethe device operation to hit a resonance band. While this device providesreasonable extinction of ˜35:1, which would be useful for manyapplications, this contrast is inadequate for applications, such asdigital cinema, which require higher performance. Furthermore, thisdevice only operates properly within narrow wavelength bands (˜25 nm)and the device is rather angularly sensitive (a 2° shift in incidenceangle shifts the resonance band by ˜30 nm). These considerations alsomake the device unsuitable for broadband wavelength applications inwhich the wire grid device must operate in “fast” optical system (suchas F/2).

Most recently, U.S. Pat. Nos. 6,108,131 (Hansen et al.) and 6,122,103(Perkins et al.), both assigned to Moxtek Inc. of Orem, Utah, describewire grid polarizer devices designed for the visible spectrum.Accordingly, U.S. Pat. No. 6,108,131 describes a straightforward wiregrid polarizer designed to operate in the visible region of thespectrum. The wire grid nominally consists of a series of individualwires fabricated directly on a substrate with a ˜0.13 μm gridlinespacing (p˜λ/5), wire nominal width of 0.052-0.078 μm wide (w), and wirethickness (t) greater than 0.02 μm. By using wires of ˜0.13 μm gridspacing or pitch, this device has the required sub-visible wavelengthstructure to allow it to generally operate above the long wavelengthresonance band and in the wire grid region. U.S. Pat. No. 6,122,103proposes a variety of improvements to the basic wire grid structuredirected to broadening the wavelength spectrum and improving theefficiency and contrast across the wavelength spectrum of use withoutrequiring finer pitch structures (such as ˜λ/10). Basically, a varietyof techniques are employed to reduce the effective refractive index (n)in the medium surrounding the wire grid, in order to shift the longestwavelength resonance band to shorter wavelengths (see equations (1) and(2)). This is accomplished most simply by coating the glass substratewith a dielectric layer which functions as an anti-reflectional (AR)coating, and then fabricating the wire grid onto this intermediatedielectric layer. The intermediate dielectric layer effectively reducesthe refractive index experienced by the light at the wire grid, therebyshifting the longest wavelength resonance shorter. U.S. Pat. No.6,122,103 also describes alternate designs where the effective index isreduced by forming grooves in the spaces between the wires, such thatthe grooves extend into the substrate itself, and/or into theintermediate dielectric layer which is deposited on the substrate. As aresult of these design improvements, the low wavelength band edge shifts˜50-75 nm lower, allowing coverage of the entire visible spectrum.Furthermore, the average efficiency is improved by ˜5% across thevisible spectrum over the basic prior art wire grid polarizer.

While the devices described in U.S. Pat. Nos. 6,108,131 and 6,122,103are definite improvements over the prior art, there are yet furtheropportunities for performance improvements for both wire grid polarizersand polarization beamsplitter. In particular, for optical systems withunpolarized light sources, where system light efficiency must bemaximized, polarization beamsplitters which provide high extinction ofboth the reflected and transmitted beams are valuable. As thecommercially available wire grid polarizers from Moxtek provide only˜20:1 contrast for the reflected channel, rather than 100:1, or moredesirable 2,000:1, its utility is limited. Additionally, the performanceof these devices varies considerably across the visible spectrum, withthe polarization beamsplitter providing contrast ratios for thetransmitted beam varying from ˜300:1 to ˜1200:1 from blue to red, whilethe reflected beam contrast ratios vary from 10:1 to 30:1. Thus thereare opportunities to provide polarization contrast performance in theblue portion of the visible spectrum in particular, as well as moreuniform extinction across the visible. Finally, there are alsoopportunities to improve the polarization contrast for the transmittedpolarization light beyond the levels provided by prior art wire griddevices. Such improvements would be of particular benefit for the designof electronic imaging systems, such as electronic projection systems,including those for digital cinema.

Thus, there exists a need for an improved wire grid polarizer,particularly for use in visible light systems requiring broad wavelengthbandwidth and high contrast (target of 1,000:1 or greater). In addition,there exists a need for such an improved wire grid polarizer for use atincidence angles of about 45 degrees.

SUMMARY OF THE INVENTION

Briefly, according to one aspect of the present invention a wire gridpolarizer for polarizing an incident light beam comprises a substratehaving a first surface. A grid or array of parallel, elongated,conductive wires is disposed on the first surface, and each of theadjacent wires are spaced apart on a grid period less than a wavelengthof incident light. Each of the wires comprises an intra-wiresubstructure of alternating elongated metal wires and elongateddielectric layers. The wires can be immersed or imbedded within anoverall structure of the wire grid polarizer, to facilitate usefuloptical devices. Design and fabrication methods for completing thesewire grid polarizer devices are also described.

Additionally, as another aspect of the present invention, improvedmodulation optical systems, comprising a polarization based reflectivespatial light modulator, which is generally a liquid crystal display(LCD), an improved wire grid polarization beamsplitter of the presentinvention, and other polarization optics, are described in variousconfigurations.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective view of a prior art wire grid polarizer.

FIGS. 2a and 2 b are plots illustrating the relative performance ofprior art wire grid polarizers and polarization beamsplitters designedto operate within the visible spectrum.

FIGS. 3a and 3 b are plots of transmitted, reflected, and overallpolarization contrast ratios versus wavelength in the visible spectrumfor a wire grid polarization beamsplitter of a type described in theprior art.

FIG. 4 is a contour plot of the overall contrast versus angle ofincidence for 500 nm light for a wire grid polarization beamsplitter ofa type described in the prior art.

FIGS. 5a-5 f are sectional views of various configurations of the wiregrid polarizer according to the present invention.

FIGS. 6a and 6 b are graphical plots illustrating a reflected andtransmitted polarization contrasts ratio versus wavelength, and theoverall contrast ratio versus wavelength for a wire grid polarizer ofthe present invention, wherein the device has a six layer structure.

FIGS. 7a-7 d are graphical plots illustrating a reflected andtransmitted polarization contrasts ratio versus wavelength, and theoverall contrast ratio versus wavelength for a wire grid polarizer ofthe present invention, wherein the device has an eighteen layerstructure.

FIGS. 8a and 8 b are graphical plots illustrating a reflected andtransmitted polarization contrasts ratio versus wavelength, and theoverall contrast ratio versus wavelength for a wire grid polarizer ofthe present invention, wherein the device has an alternate eighteenlayer structure.

FIGS. 9a and 9 b are graphical plots illustrating a reflected andtransmitted polarization contrasts ratio versus wavlength, and theoverall contrast ratio versus wavelength for a wire grid polarizer ofthe present invention, wherein the device has a five layer structure.

FIGS. 10a and 10 b are graphical plots illustrating a reflected andtransmitted polarization contrasts ratio versus wavelength, and theoverall contrast ratio versus wavelength for a wire grid polarizer ofthe present invention, wherein the device has an alternate five layerstructure.

FIGS. 11a-11 c are sectional views of various configurations ofmodulation optical systems that could utilize the wire grid polarizersaccording to the present invention.

FIGS. 12a-12 c are sectional views that sequentially illustrate thefabrication of the wire grid polarizers according to the presentinvention.

DETAILED DESCRIPTION OF THE INVENTION

Reference will now be made to the drawings in which the various elementsof the present invention will be given numerical designations and inwhich the invention will be discussed so as to enable one skilled in theart to make and use the invention.

FIG. 1 illustrates a basic prior art wire grid polarizer and definesterms that will be used in a series of illustrative examples of theprior art and the present invention. The wire grid polarizer 100 iscomprised of a multiplicity of parallel conductive electrodes (wires)110 supported by a dielectric substrate 120. This device ischaracterized by the grating spacing or pitch or period of theconductors, designated p; the width of the individual conductors,designated w; and the thickness of the conductors, designated t.Nominally, a wire grid polarizer uses sub-wavelength structures, suchthat the pitch (p), conductor or wire width (w), and the conductor orwire thickness (t) are all less than the wavelength of incident light(λ). A beam of light 130 produced by a light source 132 is incident onthe polarizer at an angle θ from normal, with the plane of incidenceorthogonal to the conductive elements. The wire grid polarizer 100divides this beam into a specularly reflected light beam 140, and anon-diffracted, transmitted light beam 150. A high order diffractedlight beam 160 could also be present, if the incident beam of light 130contains light of a wavelength that sees the wire grid structure ofwires 110 and grooves 115 as a diffraction grating rather than as asub-wavelength structure. The normal definitions for S and Ppolarization are used, such that the light with S polarization has thepolarization vector orthogonal to the plane of incidence, and thusparallel to the conductive elements. Conversely, light with Ppolarization has the polarization vector parallel to the plane ofincidence and thus orthogonal to the conductive elements.

Referring to FIG. 2a there is shown, for wavelengths within the visiblespectrum, the transmission efficiency curve 200 and the transmitted “p”polarization contrast ratio curve 205 for a commercially available wiregrid polarization beamsplitter from Moxtek Inc. of Orem, Utah. Thisdevice is similar to the basic wire grid polarization beamsplitterdescribed U.S. Pat. No. 6,108,131, which has ˜130 nm pitch (p˜λ/5) wires(parallel conductive electrodes 110) made with a 40-60% duty cycle(52-78 nm wire width (w)) deposited on a dielectric substrate 120. Thesolid metal wires are defined to be >20 nm thick, which guaranteessufficient metal thickness that the skin depth (δ) is exceeded forvisible wavelengths. This data is representative for this device for amodest NA (numerical aperture) light beam, incident on the wire gridpolarizer 100 at an angle of incidence (θ) of 45°. As this devicedivides the incident beam of light 130 into two outgoing polarized beams(140 and 150), that travel paths spatially distinguishable from theincoming light path, this device is considered to be a polarizingbeamsplitter. The transmitted contrast ratio curve 205 measures theaverage contrast of the transmitted “p” polarized light, relative to thetransmitted “s” polarized light (Tp/Ts), where the “s” polarized lightis undesirable leakage. Likewise, the reflected contrast ratio curve 210measures the average contrast of the reflected “s” polarized lightrelative to the “p” polarized light (Rs/Rp). Referring to FIG. 2b, thereis shown for wavelengths within the visible spectrum, the averageperformance for a commercially available wire grid polarizer 100 fromMoxtek for a normally incident (θ=0°) modest NA beam of light 130. Inparticular, the transmission efficiency curve 220 and the transmittedcontrast ratio curve 225, are provided (for “p” polarized light). Theperformance of both of these devices, which generally provide “p”polarization transmitted beam contrasts >300:1 is quite good, andsatisfactory for many applications.

Although the performance curves shown in FIGS. 2a and 2 b are very goodrelative to pre-existing wire grid devices, as well as pre-existingpolarizers in general, there is yet room for improvement. In particular,the contrast ratio of the reflected “s” polarized beam is rather low, asmeasured by the reflected contrast ratio curve 210, for the wire gridpolarizing beamsplitter. Polarization contrast is only 10:1 in the bluespectrum (at 450 nm), and even in the red (650 nm), it has risen only to˜40:1. In applications where both the reflected and transmitted beamsneed good polarization contrast, this performance is insufficient. As anexample, in LCD based electronic projection systems, where the projectedlight is both transmitted through and reflected off of the polarizationbeamsplitter and where the beams are fast (F/4 or less), the lowperformance in reflection will require that the system be augmented withadditional components. Additionally, while this prior art wire gridpolarization beamsplitter provides contrast ˜1200:1 in the red, thepolarization varies considerably with wavelength, and falls to ˜400:1 inthe low blue (see again transmitted contrast ratio curve 205 of FIG.2a).

The performance level of the basic wire grid polarizer can be improvedby changing the width of the wires, the thickness of the wires, thepitch of the wires, or any combination of these three. However, thesedesign changes may not necessarily provide contrast ratios desired forthe reflected beam or across the required wavelength bands. Moreover,the improvements in wire grid design performance described in U.S. Pat.No. 6,122,103, which broaden the wavelength pass band and increasetransmission efficiency by modifying the interaction of the incidentlight with the dielectric substrate 120 will also not necessarilyprovide sufficient contrast ratios for broadband visible high contrastapplications. The wire grid polarizers of U.S. Pat. Nos. 6,108,131 and6,122,103, as well as the other cited prior art wire grid device patentsonly exploit resonance effects within the plane(s) of the elongatedwires (X:Y plane of FIG. 1), which comprise the wire grid polarizer orpolarization beamsplitter. As the incident light interacts with thewires and the dielectric substrate 120 simultaneously, the structuraldetails at the interface also affect performance (as discussed in U.S.Pat. No. 6,122,103). Thus the plane of the wires should be considered toinclude the wires themselves as well as the immediate surface andsubsurface of the dielectric substrate 120.

In order to provide a benchmark for the improved devices of the presentinvention, some prior art devices were analyzed in greater detail. FIG.3a shows the calculated reflected and transmitted polarization contrastratios as a function of wavelength for a device similar to the prior artwire grid polarization beamsplitter described in U.S. Pat. No.6,108,131. This analysis was modeled using the Gsolver grating analysissoftware tool, which allows sub-wavelength structures to be thoroughlymodeled using rigorous coupled wave analysis (RCWA). Gsolver iscommercially available from Grating Solver Development Company, P.O. Box353, Allen, Tex. The wire grid device was modeled as a series ofparallel elongated wires formed directly on the transparent glasssubstrate. The analysis assumes an aluminum wire grid with period p=0.13μm, conductor width w=0.052 μm (40% duty cycle), conductor thicknesst=0.182 μm, and substrate refractive index n=1.525. For simplicity, thisanalysis only considers a collimated beam incident on the wire gridpolarization beamsplitter at an angle θ=45°. FIG. 3a provides thecollimated transmitted beam contrast 250 (Tp/Ts) and the collimatedreflected beam contrast 255 (Rs/Rp). The calculated transmitted beamcontrast 250 ranges from 10⁴-10⁵:1 across the visible spectrum, which ismuch greater than the ˜1,000:1 levels reported for the actual device, asshown in FIG. 2a. However, plot 250 of FIG. 2a represents the angleaveraged performance of an actual device, while plot 250 of FIG. 3arepresents the theoretical performance of a collimated beam though aperfect device. FIG. 3a also shows the theoretical reflected beamcontrast 255 as modeled for this prior art type wire grid devices. Thecalculated theoretical reflected beam contrast ranges from ˜10:1 to˜100:1 over the visible spectrum, and is only marginally better than thereflected beam contrast 255 given in FIG. 2a for an actual device. FIG.3b shows a plot of the theoretical overall contrast 275, where theoverall contrast C is calculated as:

C=1/((1/Ct)+(1/Cr))  (3).

The overall contrast C, which combines the contrast of the transmittedlight beam 150 (“p” polarization) with the contrast of the reflectedlight beam 140 (“s” polarization), can be seen to be mostly determinedby the lowest contrast ratio, which is the contrast for the reflectedlight beam. Thus, the overall contrast of the prior art type device perU.S. Pat. No. 6,108,131 is limited by the “s” polarization reflectedbeam, and is only ˜10:1 to ˜100:1 within the visible spectrum, with thelowest performance for blue wavelengths.

FIG. 4 shows the modeled variation of the overall contrast ratio C ascontour lines versus angle at 500 nm for this same prior art type device(0,0 coordinate corresponds to 45°). This shows that the overallcontrast ratio 275 varies significantly with incidence angle, from ˜23:1at 45° incidence, to ˜14:1 at ˜55° incidence (polar angle+10 ) to ˜30:1at ˜35° incidence (polar angle+10°, azimuthal angle 180°). Thus, FIG. 4effectively shows how the overall contrast ratio is average lower byhaving large NA incident beams of light. Of course, the overall contrastC is limited by the reflected contrast (Rs/Rp). A similar analysis ofjust the transmitted beam contrast (Tp/Ts) versus angle shows thecontrast contour lines follow a “Maltese Cross” pattern, with very highcontrast values (>10⁴:1) only in a very narrow angular range, whileaverage contrast values of ˜800:1 can be found within a fairly wide(>12°polar, 25° azimuthal) angular range. The light efficiency was alsomodeled with Gsolver, basically verifying the transmission efficiencycurve 200 of FIG. 2a. The transmission efficiency for “p” polarizedlight was fairly uniform ˜87% across most of the visible spectrum, whilethe reflected “s” light efficiency was a very uniform ˜92% across thevisible spectrum.

Wire grid polarizer 300 of the present invention, as shown as asectional view in FIG. 5a, employs a construction wherein each of theelongated composite wires 310 (or parallel conductive electrodes) has astratified internal structure comprised of a series of multipleelongated metal wires (320, 322, 324) and alternating elongateddielectric strips (dielectric layers 340, 342, 344) deposited on atransparent dielectric substrate 305. By properly constructing thecomposite wires 310 of the wire grid polarizer, with the respectivethickness of the metal wires and the dielectric layers properly defined,a combination of photon tunneling and the intra-grid resonance effectscan be exploited to enhance the performance of the polarizer. Incontrast to the prior art wire grid polarizers, the wire grid polarizersof the present invention not only uses resonance effects within theplane (X:Y plane) of the elongated wires, but also uses resonanceeffects between multiple parallel intra-wire planes along the Z axis todefine and enhance the performance. It should be understood that thewire grid polarizers 300 depicted in FIG. 5a-5 d are not to scaleillustrations, and the composite wires 310 are exaggerated to show theintra-wire substructure of elongated metal wires alternating withdielectric layers. As previously, with the prior art wire grid devices,the pitch (p) and the wire width (w) are sub-wavelength in dimension(˜λ/5 or smaller). The wire thickness (t) is also nominallysub-wavelength as well, although not necessarily so, as will bediscussed.

In particular, the design of the wire grid polarizers of the presentinvention is based upon the use of a little known physical phenomena,resonance enhanced tunneling, in which properly constructed metal layerscan be partially transparent to incident light. This phenomena, whichoccurs when a photonic band gap structure is constructed which enablesresonance enhanced tunneling, is described in the literature, forexample in a survey article “Photonic Band Gap Structure Makes MetalsTransparent” in OE Reports, December 1999, pg. 3. The concepts are alsodescribed in greater detail in the article “Transparent,Metallo-Dielectric, One-Dimensional, Photonic Band-Gap Structures” in J.App. Phys. 83 (5), pp. 2377-2383, Mar. 1, 1998, by M. Scalora et al.

Traditionally, incident light is considered to only propagate through ametal film only a short distance, known as the skin depth (δ), beforereflection occurs. Skin depth can be calculated by equation (4) asfollows:

δ=λ/4πn _(i),  (4)

where the calculated depth corresponds to the distance at which thelight intensity has decreased to ˜1/e² of its value at the input surface(where n_(i) is the imaginary part of the refractive index).Traditionally, thin metal layers are considered opaque relative totransmitted visible light when their thicknesses exceed the typical skindepth values δ, of only 10-15 nm, for metals such as aluminum andsilver. However, as these articles describe, a metallo-dielectricphotonic band gap structure can be constructed with alternating layersof thin metal sheets and thin dielectric sheets, such that the incidentlight can be efficiently transmitted through individual metal layerswhich are thicker than the skin depth δ. (By definition, a photonic bandgap structure is a nanoscopic structure with alternating layers ofmaterials or sections of similar thicknesses having different indices ofrefraction which are periodically or quasi-periodically spaced on asubstrate or other structure, such that a range of wavelengths istransmitted (or blocked) by the structure.) Most simply, thesestructures can be imagined by considering any single composite wire 310of FIG. 5a, and its constituent alternating metal wires (320, 322, 324)and dielectric layers (340, 342, 344) as being stretched into a sheet tocover much of the two dimensional surface of the dielectric substrate305. For example, one three period structure described in thesearticles, which has three 30 nm thick aluminum (Al) layers separated bythree 140 nm thick magnesium fluoride layers (MgF2), provides a variable15-50% transmission in the green wavelength band. In effect, incidentlight tunnels through the first thin metallic layer, and evanescentlyencounters the following dielectric layer. The light transmitted throughthe first metal layer into the following dielectric layer encounters thesecond metal layer. The proper boundary conditions are then establishedsuch that the overall structure acts much like a Fabry-Perot cavity (orEtalon) and resonance in the dielectric layer enhances lighttransmission through the metal layers. The resonance enhanced tunnelingeffect is then further enhanced by the repeated design of the structure,with alternating thin metallic and thin dielectric layers. Indeed, thesearticles show that adding more periods (and thus adding to the totalmetal thickness) can increase total light transmission versus structureswith fewer periods, as well as reduce the oscillations within thebandpass region. Furthermore, it is shown that adjustment of thedielectric layer thicknesses can shift the edges of the bandpassstructure towards longer or shorter wavelengths, depending on thechanges made. Typically, the thin dielectric layers in these structuresare significantly thicker than the thin metal layers (˜3-10× orgreater), while the thin metal layers may be only a skin depth thick,but may also be several times thicker than the theoretical skin depth(δ).

This resonance enhanced tunneling phenomena which is possible withmetallo-dielectric photonic bandgap has not been widely used inpractical devices. In the cited literature references, this effect isconsidered useful for light shielding devices, which transmit onewavelength band (the visible for example), while blocking nearby bands(UV and IR). Indeed, such a photonic bandgap structure can providesuppression of nearby wavelength bands which is orders of magnitudeimproved over that of a simple metallic film. Additionally, U.S. Pat.No. 5,751,466 (Dowling et al.) and U.S. Pat. No. 5,907,427 (Scalora etal.) describe use of this effect to design variable photonic signaldelay devices for optical telecommunications. However, the prior artdoes not foresee the benefit of applying the resonance enhancedtunneling of metallo-dielectric photonic bandgap structures to thedesign of polarization devices generally, or to wire grid polarizers andpolarization beamsplitters in particular. Moreover, it is notnecessarily clear that the resonance enhanced tunneling effect wouldimprove the performance of a wire grid polarization device by improvingpolarization contrast or transmission across the entire visiblespectrum, or even any single color band.

Accordingly, the wire grid polarizers 300 of the present invention, asdepicted in FIGS. 5a-5 d, use a plurality of identically fabricatedelongated composite wires 310, each with an intra-wire substructurecomprising alternating metal wires (320, 322, 324) and dielectric layers(340, 342, 344). As with the prior art wire grid polarizers, light ofthe polarization parallel to the wires is reflected off the device, andlight of polarization orthogonal to the wires is transmitted. However,where the prior art wire grid polarizers use relatively thick wires, ofmonolithically deposited metal typically 100-150 nm thick, the wire gridpolarizers of the present invention effectively constructs each wire asa series of alternating thin metal layer and dielectric layers. As aresult, the incident light of polarization orthogonal to the wires istransmitted in part through the metallic layers themselves by photonictunneling and enhanced resonance effects, and thus the overall contrastratio of the transmitted polarized light versus the reflected polarizedlight is enhanced. As compared to the prior art wire grid polarizationdevices, which rely only on resonance effects within the plane of thewires (the X:Y plane of FIG. 1), the wire grid polarization devices ofthe present invention also use resonance effects in the orthogonaldirection (the Z direction of FIG. 1) to determine the performance.

The first example of a wire grid polarizer 300 of the present inventionis shown in FIG. 5a, wherein each elongated composite wire 310 has aperiodic stratified intra-wire structure 315 of six layers comprisingalternating layers of metal (metal wires 320, 322, 324) and dielectric(dielectric layers 340, 342, 344). As with the prior art devices, wiregrid polarizer 300 was modeled as a structure with the wires located ona 130 nm pitch (p˜λ/5), with a duty cycle of 40%, such that the width(w) of the wires is 52 nm. Thus, grooves 312 between composite wires 310are 78 nm wide. Grooves 312 are nominally filled with air, rather thansome other medium, such as an optical liquid or gel. Likewise, as withthe prior art type device, this device was modeled as a polarizationbeamsplitter, with a collimated beam incident at an angle θ=45°.Additionally, composite wires 310 were modeled with an intra-wirestructure 315 comprising three thin dielectric layers (dielectric layers340, 342, 344) of MgF2, each 33 nm thick, alternating with three thinmetal layers (metal wires 320, 322, and 324) of aluminum, each 61 nmthick.

According to the effective medium theory, incident light interacts withthe effective index of each layer, where the effective index depends onthe geometry of the composite wires 310, the geometry of the layeritself, the complex refractive index of the layer (either metal ordielectric), the refractive index of the material between the wires(air), and the boundary conditions established by the adjacent layers.As shown in FIG. 5a, for this example of wire grid polarizer 300, theintra-wire structure is designed such that the third dielectric layer344 is located in between the third metal wire 324 and surface 307 oftransparent dielectric substrate 305. The total wire thickness (t) ofthe composite wires 310, which is the sum of the thicknesses of thethree metal wires 320, 322, and 324 and the three dielectric layers 340,342, 344, is 282 nm or (˜λ/2). The modeled polarization performance forthis device, which is shown in FIGS. 6a and 6 b, is an improvement inboth reflection and transmission to the basic wire grid polarizer whosemodeled results were given in FIGS. 3a and 3 b. Performance was modeledwith Gsolver, using 8 diffraction orders, to ensure accuracy. As shownin FIG. 6a, the theoretical transmitted beam contrast 250 for “p” lightvaries from 10⁵-10⁶:1 across the visible spectrum, while the reflectedbeam contrast 255 for “s” light averages a fairly uniform ˜100:1 acrossthe visible spectrum. Thus, the overall contrast ratio 275, shown inFIG. 6b, also averages ˜100:1 across the entire visible spectrum. Theimproved polarization performance is not gained at the cost ofefficiency, as the “s” light reflection efficiency is ˜91%, while the“p” light transmission efficiency is ˜83%, with little variation acrossthe visible spectrum. With such a relatively high and uniformpolarization contrast for the reflected “s” polarization light, thisdevice could provide improved performance as a polarizationbeamsplitter, in applications where both “p” and “s” polarized beams areto be used. Notably, this device also shows a ˜10× improvement in the“p” polarized light contrast (also known as polarization extinctionratio) over the prior art device of U.S. Pat. No. 6,108,131, as well asan enhanced blue performance, with the reflected beam contrast 255 andthe overall contrast ratio 275 averaging ˜250:1 contrast over most ofthe blue spectrum. Such performance could be useful in manyapplications, including projection systems.

Additionally, the improvements in overall contrast 275 and transmittedbeam contrast 250 of the first example wire grid polarizationbeamsplitter device, as shown in FIGS. 6a,b, when compared to the priorart type device, as shown in FIGS. 3a and 3 b, do not come at the costof reduced angular performance. A contour plot analysis of the overallcontrast C showed that average contrast values of 500:1 are obtainedwithin a wide angular swath (+/−12° polar, and +/−30° azimuthal) at 500nm. This first example device was also modeled for a collimated beam ata normal incidence (θ=0°). As the transmitted beam contrast over theentire visible spectrum >105:1 at normal incidence, the first examplewire grid polarizer was proven to function well as a polarizationanalyzer or polarizer, and not just as a wire grid polarizationbeamsplitter.

While both the present invention for a wire grid polarizer and the wiregrid polarizer of Garvin et al. in U.S. Pat. No. 4,289,381, both havemultiple planes of patterned wires extending in the Z axis direction,these wire grid polarizer devices are distinctly different. Inparticular, the wires in each of the multiple wire grid planes of U.S.Pat. No. 4,289,381 are thick (100-1000 μm) solid metal wires, which lackintra-wire substructure and which are too thick for useful evanescenttransmittance through the wires. Additionally, the multiple wire planesfor the two grid case of U.S. Pat. No. 4,289,381 preferentially have ahalf pitch offset (p/2) rather than having an overlapped alignment.Finally, the U.S. Pat. No. 4,289,381 wire grid polarizer designpreferentially locates adjacent wire grids with an inter-grid spacing(1) and pitch offset (p/2) so as to avoid the occurrence of inter-gridresonance or Etalon effects. In contrast, the wire grid polarizers 300of the present invention specifically use Etalon type resonance effectswithin stratified intra-wire substructure in order to enhanceperformance.

The second example of a wire grid polarizer 300 of the present inventionis shown in FIG. 5b, wherein each composite wire 310 has a periodicstratified intra-wire structure 315 of eighteen layers comprisingalternating layers of metal (metal wires 330 a-i) and dielectric(dielectric layers 350 a-i). As with the first example device, thesecond example wire grid polarizer 300 was modeled as a structure 130 nmpitch (p˜λ/5) composite wires 310, with a 40% duty cycle wire width (w)of 52 nm. Likewise, as before, the device was modeled as a polarizationbeamsplitter, with a collimated beam incident at an angle θ=45°. Aspreviously, the final dielectric layer (330 i) is adjacent to thedielectric substrate 305. However, composite wires 310 were modeled withan intra-wire structure 315 comprising nine thin MgF2 dielectric layers(dielectric layers 330 a-i), each 39 nm thick, alternating with ninethin aluminum metal layers (metal wires 350 a-i), each 17 nm thick. Thetotal wire thickness (t) of composite wires 310, which is the sum ofthicknesses of metal wires 330 a-i and dielectric layers 350 a-i is 504nm, which is ˜1λ. The modeled polarization performance for this device,which is shown in FIGS. 7a and 7 b, is an improvement in both reflectionand transmission to the basic wire grid polarizer whose modeled resultswere given in FIGS. 3a and 3 b. As shown in FIG. 7a, the theoreticaltransmitted beam contrast 250 for “p” light varies from 10⁷-10⁸:1 acrossthe visible spectrum, while the reflected beam contrast 255 for 's“lightaverages ˜100:1 across the visible spectrum. Thus, the overall contrastratio 275, shown in FIG. 7b, also averages ˜100:1 across the entirevisible spectrum. While this device is significantly more complicatedthan the first example device, the theoretical transmitted beam contrast250 for “p” polarized light is ˜100× better than the first exampledevice, and ˜1,000× better than the prior art type device (see FIG. 3a).

The third example of a wire grid polarizer 300 of the present inventionis an eighteen layer structure similar to that of the second example,shown in FIG. 5b, with each composite wire 310 having a periodicstratified intra-wire structure 315 of eighteen layers comprisingalternating layers of metal (metal wires 330 a-i) and dielectric(dielectric layers 350 a-i), except that the thicknesses of thedielectric and metal layers have been changed. In this case, compositewires 310 were modeled with an intra-wire structure 315 comprising ninethick MgF2 dielectric layers (dielectric layers 330 a-i), each 283 nmthick, alternating with nine thin aluminum metal layers (metal wires 350a-i), each 17 nm thick. The total wire thickness (t) of composite wires310 is 2700 nm, which is ˜5λ. As shown in FIGS. 7c and 7 d, as comparedto FIGS. 7a and 7 b, the third device has significantly differentpolarization performance as compared to the second device, although theonly change was in the thickness of the dielectric layers 350 a-i. Asevident in FIG. 7d, the overall contrast ratio 275 has an averagecontrast ratio in the blue spectrum of ˜150:1, while performance in thegreen and red spectra have degraded. The plot of overall contrast ratio275 is also noteworthy for its rapid oscillations in the blue wavelengthband, which swing, peak to valley, between ˜50:1 and ˜500:1 contrast.This example, which uses thick dielectric layers, suggests that thepotential to design wavelength band tuned wire grid polarizationbeamsplitters which have not only excellent performance for the “p”transmitted light, but very good performance (250:1 or better) for the“s” reflected light. Unfortunately, while Gsolver is a superior analysissoftware program, the code was not written to facilitate polarizationcontrast optimization, so an exemplary result with further improvedperformance is not available. However, optimization of this design,allowing the thicknesses of the metal layers and the dielectric layersto vary, creating aperiodic or doubly periodic structures, could boostthe performance further in the blue, to provide the desired result.

It should be noted that similar results to the third example design of awire grid polarizer 300 can be obtained using similar intra-wirestructures 315 with thick dielectric layers, but with other thaneighteen total layers. The fourth example, not shown, wire gridpolarizer was modeled with a structure comprising eight layers, whereinfour layers of MgF2, each 525 nm thick, alternate with four layers ofaluminum, each 45 nm thick. Thus the total thickness (t) of thecomposite wires 310 is 2.28 μm, or ˜4λ. The modeled device is otherwisethe same as the devices of the prior examples, relative to wire pitch(p), wire width (w), and angle of incidence. The resulting polarizationperformance for this fourth example device, as shown in FIGS. 8a and 8b, is very similar to that of the third example device (FIGS. 7c and 7d) in blue spectrum. Interestingly, FIG. 8a suggests the potential for astructure with a high contrast in the blue and red spectra for both thetransmitted and reflect beams, while giving low contrast for both beamsin the green spectrum.

Relative to the second and third examples of eighteen layer wire gridpolarizers, which only vary in design according to the thickness of thedielectric layers (39 nm versus 283 nm), other interesting results canbe obtained by modeling similar devices with intermediate dielectriclayer thicknesses. For example, a modeled device with 56 nm dielectriclayer thicknesses provides a minimum ˜100:1 overall contrast ratio overthe entire visible spectrum, but also provides two localized peaks, at˜450 nm and ˜610 nm, where overall polarization contrast is ˜1000:1 orgreater.

The fifth example of a wire grid polarizer 300 of the present 10invention is shown in FIG. 5c, where each composite wire 310 has aperiodic stratified intra-wire structure 315 of five layers comprisingalternating layers of metal (metal wires 320, 322, and 324) anddielectric (dielectric layers 340 and 342). As with the other exemplarydevices, the fifth example wire grid polarizer 300 was modeled as astructure 130 nm pitch (p˜λ/5) composite wires 310, with a 40% dutycycle wire width (w) of 52 nm. Likewise, as before, the device wasmodeled as a polarization beamsplitter, with a collimated beam incidentat an angle θ=45°. However, this device has an intra-wire structure 315which is designed with a metal layer (metal wire 324) adjacent to thedielectric substrate 305, rather than a dielectric layer as in theprevious examples. Composite wires 310 were modeled with an intra-wirestructure 315 comprising two thin MgF2 dielectric layers (dielectriclayers 340 and 342, each 55 nm thick, alternating with three thinaluminum metal layers (metal wires 320, 322, and 324), each 61 nm thick.The total wire thickness (t) of composite wires 310 is 293 nm, which isλ/2. Although the modeled polarization performance for this device,which is shown in FIGS. 9a and 9 b, is an improvement in both reflectionand transmission to the basic wire grid polarizer (shown in FIGS. 3a and3 b), this five layer device does not perform as well as the six layerdevice of the first example. As shown in FIG. 7a, the theoreticaltransmitted beam contrast 250 for “p” light varies from 10⁵-10⁶:1 acrossthe visible spectrum, while the reflected beam contrast 255 for 's”light averages only ˜40:1 across the visible spectrum. Thus, the overallcontrast ratio 275, shown in FIG. 7b, also averages ˜40:1 across theentire visible spectrum. Additionally, the blue performance is lessuniform across its wavelength band, as compared to the first exampledevice. Nonetheless, this device, with a metal layer (wire 324) incontact with the dielectric substrate 305, is still useful.

The sixth example of a wire grid polarizer 300 of the present invention,as shown in FIG. 5d, is a variation of the fifth example device whichhas only five layers within each composite wire 310, where the sixthexample device has an aperiodic stratified intra-wire structure 315.Thus, composite wires 310 were modeled with an intra-wire structure 315comprising three thin aluminum metal layers (metal wires 320, 322, and324), each 61 nm thick, alternating with two thin MgF2 dielectriclayers, where dielectric layers 340 is 27.5 nm thick, while dielectriclayer 342 is 82.5 nm thick. As before, the third metal layer 324 is incontact with the dielectric substrate 305. As with the fifth exampledevice, the total wire thickness (t) for this device is 293 nm. Themodeled performance of this device, as shown in FIGS. 10a and 10 b, issimilar to that of the fifth example device (see FIGS. 9a and 9 b),except that the performance in the blue spectrum is higher on average,as measured by the overall contrast 275. The fifth and sixth exampledevice are again suggestive of the potential for wavelength band tunedwire grid polarizer devices.

Graphs of light efficiency, as measured by the “s” polarizationreflection efficiency and “p” polarization transmission efficiency forthe various examples (one to six) were not provided, as the data changedminimally. In general, the reflection efficiency for “s” polarized lightwas uniform across the visible spectrum, at levels in the upper 80's tolower 90's for percent efficiency. The “p” polarization transmissionefficiency was a bit less uniform, as some exemplary devices showed somefall-off in the low blue region of the spectrum. Also, the overall “p”polarization transmission efficiency was lower than the “s” lightefficiency, and generally was in the lower to middle 80's for percentefficiency.

It should be understood that each elongated composite wire 310 has alength that is generally larger than the wavelength of visible light.Thus, the composite wires 310 have a length of at least approximately0.7 μm. However, in most practical devices, the composite wires 310 willbe several millimeters, or even several centimeters in length, dependingon the size requirements of the application. While the various exemplarywire grid polarizer devices of the application are modeled with a dutycycle of 40% relative to the width (w) of the composite wires 310 ascompared to grid pitch or period (p), it should be understood that otherduty cycles can be used. Generally, duty cycles in the range of 40-60%will provide the optimum overall performance relative to transmissionand contrast ratio. It is noteworthy, as illustrated by the exemplarydevices, that the total thickness (t) of the composite wires 310 canvary from approximately a half-wave to approximately five waves whilestill providing exceptional transmission of the transmitted “p”polarized light and rejection of the “s” polarized light. On the otherhand, current device fabrication process methods may limit theachievable aspect ratio (thickness (t) to width (w)) for the compositewires 310. As a result, practical devices in the visible spectrum may belimited to total thicknesses (t) of only ˜100-300 nm range (˜λ/6 to˜λ/2). Thus, a total wire thickness limitation could constrain thesolution space and design freedom for possible designs based oncomposite wires 310 with a stratified intra-wire structure 315 ofalternating metal wire and dielectric layers, but nonetheless, even in alimited solution space, advantaged designs can be found. By comparison,prior art wire grid devices largely rely on the thickness of the metalwires being thicker than several skin depths (δ) in order to ensure goodrejection of the “s” polarized light. Furthermore, it is noteworthy thatthe exemplary devices of this application may have thicknesses of theelongated metal wires (330, for example), which are only several(approximately 1-4) skin depths thick and still provide exceptionaltransmission of the transmitted “p” polarized light and rejection of the“s” polarized light. For example, the fifth example device uses multiplemetal layers each of which are 61 nm thick, which is equivalent toapproximately four skin depths. Finally, the second or opposite surfaceof the dielectric substrate 120 could have a anti-reflection (AR)coating to enhance overall transmission.

It should be understood that these various examples for designs of wiregrid polarizers 300 with stratified intra-wire grid structures 315comprising multiple alternating metal and dielectric layers do notencompass the entire range of possible designs. For one thing, thelimitations of the Gsolver software, which does not allow optimizationof polarization contrast, constrained the presented results to less thantheir potential. Also, other combinations of materials could be used inthe designs, including replacing aluminum with gold or silver, orreplacing dielectric material MgF2 with SiO2 or TiO2, for example.Actual materials choices will depend both on the desired designperformance as well as process constraints. It should also be understoodthat although all the exemplary devices were designed with the outmost(furthest from the dielectric substrate 305) layer which comprises aportion of the intra-wire structure 315 of composite wires 310 as ametal layer, that alternately a dielectric layer could be used as theoutmost layer.

Additionally, it is possible to design devices where grooves 312 arefilled with an optically clear liquid, adhesive, or gel, rather thanwith air. This is illustrated in FIG. 5e, where an optical material ofrefractive index n_(i) is a dielectic fill 360 which is formed in thegrooves 312. For example, the dielectric fill 360 may be the samedielectric material that is used to form the dielectric layers 340, 342,344, which partially comprise composite wires 310. The resulting wiregrid polarizer 300 is effectively immersed, with the benefit that thecomposite wires 310 are protected (from oxidation, for example) by thedielectric fill 360.

A fully imbedded wire grid polarizer is depicted in FIG. 5f, where thecomposite wires 310 are not only fabricated on dielectric substrate 305,but are also overlaid and in contact (or nearly so) with a seconddielectric substrate 355, to form an integrated device containing aninternal polarizing layer. In this case, the integrated device is aplate polarizer. The two dielectric substrates may or may not haveidentical optical properties. Preferably an intervening optical materialof index n_(i) (dielectric fill 360) is provided, which could comprisean optically clear adhesive (or epoxy or gel), that fills the grooves312 and contacts the second dielectric substrate 355, helping to secureit against the structure of composite wires 310. Alternately, thedielectric fill 360 could consist of the same dielectric materials usedto form the dielectric layers 340, 342, and 344, while a separateoptical material, such as an epoxy, is used to secure the seconddielectric substrate 355 against the structure of composite wires 310.To avoid internal reflection problems, the intervening optical materialused for the dielectric fill 360 in the wire grid polarizers of FIGS. 5eand 5 f likely is either index matched to the dielectric substrates(n_(i)=n_(d)) or has a somewhat lower refractive index (n_(i)<n_(d)).This intervening optical material may also overcoat the composite wires310 with a thin layer, rather than having the wires contact the secondsubstrate directly. Unfortunately, filling the grooves with a dielectricmaterial of an index n_(i) other than air, will degrade the designperformance of the wire grid device, by lower contrast and shifting upthe low wavelength band edge. These changes can be compensated for byalternate designs for the composite wires 310, which could change boththe wire pitch (p) and the intra-wire substructure 315.

Notably, the dielectric substrates of imbedded wire grid polarizer ofFIG. 5f could each be right angle prisms, where the polarizing layer isfabricated on the hypotenuse. The prisms can then be assembled to forman integrated polarizing prism. In this case, the polarizing layer liesalong the internal diagonal, thereby providing an equivalent to aMacNeille type prism, but presumably having enhanced performance.

As another point, it should be noted that the exemplary devices featureonly one device structure with an aperiodic structure. While that device(the sixth example) is relatively simple, much more complicated devicesare possible, depending both on the ability to optimize the design andto fabricate the device. The thicknesses of both the metal layers andthe dielectric layers that comprise the stratified intra-wiresubstructure 315 can be varied through the structure. For example,quasi-periodic intra-wire structures, such as chirped structures, couldbe designed. As another example, the intra-wire structure 315 could bedesigned to periodically alternate the metal and dielectric layers,except for tuning the thickness of the outermost layer and/or the inmostlayer (closest to the dielectric substrate 305), to improve theperformance across the interfaces to regions outside the grid. Likewise,dielectric substrate 305 could be coated with an intermediate layer,with the inmost layer of the intra-wire structure of composite wires 310in direct contact with the intermediate layer, rather than with thedielectric substrate 305. Of course, device optimization not onlydepends on the details of the intra-wire structure 315, but also on thewire pitch (p) and the wire width (w). In effect, the concepts ofdesigning a wire grid polarizer 300, composed of composite wires 310with stratified intra-wire structures 315, allow the wire grid device toattain performance levels otherwise provided by smaller pitchstructures.

Also, wire grid polarizer 300 could be designed and fabricated with thecomposite wires 310 having stratified intra-wire structures 315 whichvary across the surface of the device. Thus it would be possible tocreate a spatially variant device for polarization beamsplitting orpolarization analysis.

The actual process of designing a wire grid polarizer 300 (orpolarization beamsplitter) with the stratified intra-wire substructure315 comprising multiple metal layers and alternating dielectric layers,as generally depicted in FIGS. 5a-5 f, begins with a definition of thespecifications for the device. The primary specifications are thespectral bandwidth, angle of light incidence, angular width of theincident light (numerical aperture), the transmission efficiency (“p”pol.,>˜80%), the transmitted contrast (>1,000:1 for example), thereflection efficiency (“s” pol.,>˜80%), and the reflected contrast(>200:1 for example). The standard wire grid features, wire pitch (p)and wire width (w) are determined, with the minima set by the limitingresolution of the manufacturing process. A nominal stratified intra-wirestructure 315 for the composite wires 310, including the number andthickness of the metal layers, and the number and thickness of thedielectric layers is also defined. Other parameters, such as materialschoices for the metals and dielectrics, the materials choice for anymaterials in the grooves 312, and the periodicity or aperiodicity of theintra-wire structure 315 also are defined. Some parameters, such as wirepitch (p), wire width (w), total wire thickness, and the number oflayers, may have upper and lower boundary conditions, inside of which,iterative change can occur to drive optimization. The design process canthen proceed, assessing the results against the performance targets andmanufacturability issues.

Although wire grid polarizer 300, composed of composite wires 310 withstratified intra-wire structures 315, can have a fairly complicateddesign, the complexity does not necessarily equate to a difficultfabrication process. In general, the tolerances for fabrication of theindividual layers, whether metal or dielectric, are relatively loose.Typical layer thickness tolerances are expected to be severalnanometers, with some devices having layer tolerances over 10 nm, whileothers have 1 nm tolerances or less, depending on the design.

The process for forming the described wire grid structures can beaccomplished in several ways. In the case of composite wires 310 withair filling the grooves 312 between the wires, construction begins witha dielectric substrate 305. In the following example, a first metallayer is deposited using a metal such as aluminum. The deposition methodcan be one of several standard methods including thermal evaporation orsputtering. Next the metal is patterned using standard photolithographyfollowed by a metal etch (possibly dry metal etch such as CC14, BC13),to form a first layer of metal wires 330 c. This is shown in FIG. 12a.Next, a first dielectric layer 350 c is deposited. Suggested dielectricsinclude, but are not limited to SiO2 and MgF2. Depending on theequipment used and the number of layers expected, it may be necessary toplanarize after a dielectric layer is deposited. Another metal layer(330 b) is placed and patterned as before, followed by a dielectriclayer (350 b), and possibly planarization, to form a metal/dielectricstack. This process is repeated until the metal/dielectric stack hasachieved the desired number of layers to form the composite wires 310.Each layer can be designed and controlled with dimensions different fromthe previous layer. FIG. 12b shows the completed stack of alternatingmetal wires 330 and dielectric layers 350. Note that a dielectric fill360 has collected in the grooves 312 during the process of patterningthe composite wires 310. In the case where the final layer is metal,after the last metal layer has been patterned, a dielectric etch is usedto remove the dielectric fill 360 from the grooves 312 between thecomposite wires 310. (A possible dry etch of oxide and nitride caninclude CF4, and SF6.) In the case where the final layer is dielectric,a masking layer is patterned on the final layer of dielectric, theinterwire dielectric fill material 360 is removed, and then the maskinglayer is removed. The final device is shown in FIG. 12c. If the firstlayer deposited on the dielectric substrate 305 is a dielectric, it maybe deposited first, then the metal pattering begins.

An alternative structure with the interwire region (grooves 312) filledwith dielectric follows an even simpler fabrication process. The methodis as before, except the dielectric is not removed from the interwireregions. This method has the advantage that there is no potential fordamage to the composite wires 310 from a dielectric etch process toremove the dielectric fill 360.

Alternatively methods including repeatedly etching the dielectric or ionbeam milling could be employed. Also, one could consider lift offmethods. If one considers wet etch, particularly if etching follows eachlayer deposition possible choices of etch chemicals include HF for aSiO2 etch, and PAN for an Aluminum etch. It should be understood thatthere are many ways to fabricate this device. The choice of methoddepends on materials used and particular stack structure.

As previously discussed, the wire grid polarizers 300 of the presentapplication, with the stratified intra-wire grid structures havingmultiple metal layers alternating with dielectric layers, can provideimproved performance both as normal incidence polarizers and as off axisincidence polarization beamsplitter. The design and use of thisstructure as a polarization beamsplitter has considerable attraction,due to the potential to significantly enhance the reflected contrast(Rs/Rp). Accordingly, FIG. 11a depicts a modulation optical system 400,comprising a wire grid polarization beamsplitter 410 and a reflectivespatial light modulator 445, where the incident beam of light 130 splitsinto a reflected light beam 140 and a transmitted light beam 150, thelatter of which illuminates the modulator. Spatial light modulator 445,which is part of a modulator assembly 440 that includes mounting,cooling, and an electronics interface (all not shown), is nominally anLCD (liquid crystal display) which modulates the polarization state ofthe incident transmitted beam of light 150. An image bearing light beam490 is created by the process of modulation and reflection from the LCD445, and subsequent reflection off the wire grid polarizationbeamsplitter 410. The light in the reflected light beam 140 can bedirected into a light trap (not shown). The effective optical path totaking by imaging light encompasses incident beam of light 130,transmitted light beam 150, and image bearing light beam 490.

In accordance with the present invention, the wire grid polarizationbeamsplitter 410 comprises a structure of stratified sub-wavelengthwires 430 and grooves 435, with composite wires having a structure ofmultiple metal layers alternating with dielectric layers, formed on asecond surface 420 of dielectric substrate 425. As depicted in FIG. 11a,the wire grid polarization beamsplitter 410 is preferably oriented withthe stratified sub-wavelength wires 430 closest to the LCD 445, whilethe first surface 415 (which is nominally AR coated) faces the incidentbeam of light 130. By locating the second surface 420 bearing thestratified sub-wavelength wires 430 closest to the LCD 445, thepotential for a contrast loss due to thermally induced stressbirefringence (from absorbed light) in the wire grid polarizationbeamsplitter 410 is reduced.

The modulation optical system 400 of FIG. 11a is also depicted asincluding a pre-polarizer 470, a polarization analyzer 475, and twopolarization compensators 450 and 460. Both the pre-polarizer 470 andthe polarization analyzer 475 can be wire grid polarizers, including ofcourse, being wire grid polarizers with stratified sub-wavelength wiresin accordance with the present invention. These various components mayor may not be included, depending on the design targets and constraints.Other components, such as a projection lens (not shown), may interactwith this system.

A second exemplary modulation optical system 400 is depicted in FIG.11b, where the reflected light beam 140 illuminates the reflectivespatial light modulator (LCD) 445. The reflective spatial lightmodulator 445 then rotates the polarization state of the incident lightin accordance with the applied control signals, to impart image data tothe light. An image bearing light beam 490 is then created as the lightis transmitted through the wire grid polarization beamsplitter 410. Theeffective optical path to taking by imaging light encompasses incidentbeam of light 130, reflected light beam 140, and image bearing lightbeam 490. As the wire grid polarization beamsplitter 410 of FIG. 11bcomprises in part a tilted plate, an image of LCD 445 that is imaged bya projection lens (not shown) would suffer the aberrations (coma &astigmatism) that are induced by imaging in transmission through atilted parallel plate dielectric. These aberrations can be opticallycorrected by a variety of means. Alternately, the wire grid polarizationbeamsplitter 410 can be modified to comprise an imbedded wire gridpolarization beamsplitter, with the polarizing interface containedwithin a cube prism. Such a prism was described previously.

A third exemplary modulation optical system 400 is depicted in FIG. 11c.In this system, the two modulators, LCDS 446 a and 446 b, arerespectively illuminated by the reflected light beam 140 and thetransmitted light beam 150. Image bearing light beam 490 is created incombination, utilizing modulated (polarization rotated) light from LCD446 a which is transmitted through wire grid polarization beamsplitter410 and modulated (polarization rotated) light from LCD 446 b which isreflected from wire grid polarization beamsplitter 410. The resultingimage bearing light beam, which possesses light of both polarizations(“p” and “s” ), is imaged by project lens 495 to a target plane (such asa screen), which is not shown. By equipping modulation optical system400 of FIG. 11c with two LCDs 445 (one per polarization), the lightefficiency of the overall optical system can be enhanced. As an example,a full color system could be developed, where light is split into threecolor channels (red, green, blue), and each color channels is comprisedof a modulation optical system 400 per FIG. 11c, effectively providing asix LCD system. This option can be considered, because the wire gridbeamsplitter 410 of the present invention, with its stratified wirescomprising multiple metal wires alternating with dielectric layers,simultaneously can provide high contrast both in transmission andreflection. As previously, an alternate wire grid polarizationbeamsplitter 410 with an imbedded construction forming a cube prism canbe used to avoid tilted plate aberrations, and to provide identicaloptical path lengths for the two imaged light beams.

Other exemplary modulation optical systems can be constructed using thewire grid polarizers 300 of the present application, with the compositewires formed as stratified intra-wire grid structures having multiplemetal layers alternating with dielectric layers. For example, analternate modulation optical system, utilizing a transmissivepolarization rotating spatial light modulator, can be constructed wherethe modulator is located in an optical path between two polarizers (thepre-polarizer and the analyzer), at least one of which is a wire gridpolarizer in accordance with the present invention. These wire gridpolarizers can be used in combination with types of polarizationaltering spatial light modulators other than just liquid crystaldisplays (LCDs). For example, they can be used in conjunction with PLZTmodulators. It should be understood that any of the described modulationoptical systems 400 can be used as a sub-assembly of some larger system,such as an image printer or a projection display.

Finally, it should be noted that while this concept for an improved wiregrid polarizer 300, composed of composite wires 310 with stratifiedintra-wire structures 315, has been discussed specifically with regardsto operation in the visible spectrum, with application for electronicprojection, the concept is fully extendable to other applications andother wavelength bands. Although many of the examples focussed onenhancing the reflected contrast across the entire visible spectrum, oracross the blue spectrum, it is of course possible to enhance thereflected contrast for the other color bands individually (green andred). Indeed, the performance enhancements in the green and red spectracan be more dramatic than in the blue spectrum. Alternately, suchdevices could be designed and fabricated at near infrared wavelengths(˜1.0-1.5 μm) for use in optical telecommunication systems, or in thefar infrared (such as 20 μm), or in the electromagnetic spectrumgenerally. The concept also has the potential to produce narrowwavelength polarization devices where the “p” transmitted contrast is>10⁸:1, and where the “s” reflected contrast is >10⁴:1. Likewise, anarrow wavelength notch polarization beamsplitter could be designed, ineither the visible or infrared spectra for example, where the “p” and“s” polarization discrimination were simultaneously optimized to producea polarization beamsplitter with superior overall contrast. Also, thethird example device suggests a polarization filter device with astructure which can provide large wavelength bands with highpolarization contrast, surrounding an intermediate wavelength band whichprovides minimal polarization contrast (see FIG. 8a).

For example, such a device could be useful in a product assembly linefor quality and defect inspection, when combined with properlystructured illumination.

The invention has been described in detail with particular reference tocertain preferred embodiments thereof, but it will be understood thatvariations and modifications can be effected within the scope of theinvention.

PARTS LIST

100. Wire grid polarizer

110. Parallel conductive electrodes (wires)

115. Grooves

120. Dielectric substrate

130. Beam of light

132. Light source

140. Reflected light beam

150. Transmitted light beam

160. Diffracted light beam

200. Transmission efficiency curve

205. Transmitted contrast ratio curve

210. Reflected contrast ratio curve

220. Transmission efficiency curve

225. Reflected contrast ratio curve

250. Transmitted beam contrast

255. Reflected beam contrast

275. Overall contrast ratio

300. Wire grid polarizer

305. Dielectric substrate

307. Surface

310. Composite wires

312. Grooves

315. Intra-wire structure

320. Metal wires

322. Metal wires

324. Metal wires

330 a-i. Metal wires

340. Dielectric layers

342. Dielectric layers

344. Dielectric layers

350 a-i. Dielectric layers

355. Second dielectric substrate

360. Dielectric fill

400. Modulation optical system

410. Wire grid polarization beamsplitter

415. First surface

420. Second surface

425. Dielectric substrate

430. Stratified sub-wavelength wires

435. Grooves

440. Modulator assembly

445. Spatial light modulator (LCD)

446 a. Spatial light modulator (LCD)

446 b. Spatial light modulator (LCD)

450. Polarization compensator

460. Polarization compensator

470. Pre-polarizer

475. Polarization analyzer

490. Image bearing light beam

495. Project lens

What is claimed is:
 1. An immersed wire grid polarizer for polarizing anincident light beam, comprising: a substrate having a surface; an arrayof parallel, elongated, composite wires with intervening groovesdisposed on said surface, wherein each of said composite wires arespaced apart at a grid period less than a wavelength of said incidentlight; wherein each of said grooves is filled with a dielectric opticalmaterial; wherein each of said composite wires comprises an intra-wiresubstructure of alternating elongated metal wires and elongateddielectric layers; and wherein said intra-wire substructure ofalternating elongated metal wires and elongated dielectric layerscomprises at least two of said elongated metal wires.
 2. An immersedwire grid polarizer according to claim 1 wherein said wire gridpolarizer is oriented at an angle relative to said incident light beamsuch that said wire grid polarizer functions as a polarizationbeamsplitter and separates a transmitted polarized beam and a reflectedpolarized beam from said angle of said incident light beam.
 3. Animmersed wire grid polarizer according to claim 1 wherein saidintra-wire substructure of alternating elongated metal wires andelongated dielectric layers supports resonance enhanced tunnelingthrough said elongated metal wires, thereby enhancing transmission ofthe light of the polarization state orthogonal to said array ofcomposite wires.
 4. An immersed wire grid polarizer according to claim 1wherein said intra-wire substructure of alternating elongated metalwires and elongated dielectric layers comprises at least one of saiddielectric layers.
 5. An immersed wire grid polarizer according to claim1 wherein said incident light is within the range of approximately 0.4to 1.6 μm in the electromagnetic spectrum.
 6. An imbedded wire gridpolarizer for polarizing an incident light beam, comprising: a firstdielectric substrate having a surface; a second dielectric substratehaving a surface; an array of parallel, elongated, composite wires withintervening grooves are disposed on said surface of said firstdielectric substrate, wherein each of said composite wires are spacedapart at a grid period less than a wavelength of said incident light;wherein said array of parallel, elongated composite wires comprise apolarizing layer located between said surface of said first dielectricsubstrate and said surface of said second dielectric substrate; whereineach of said composite wires comprises an intra-wire substructure ofalternating elongated metal wires and elongated dielectric layers; andwherein said intra-wire substructure of alternating elongated metalwires and elongated dielectric layers comprises at least two of saidelongated metal wires.
 7. An imbedded wire grid polarizer according toclaim 6 wherein a dielectric optical material fills said grooves andcontacts said surface of said second dielectric substrate.
 8. Animbedded wire grid polarizer according to claim 6 wherein said wire gridpolarizer is oriented at an angle relative to said incident light beamsuch that said wire grid polarizer functions as a polarizationbeamsplitter and separates a transmitted polarized beam and a reflectedpolarized beam from said angle of said incident light beam.
 9. Animbedded wire grid polarizer according to claim 6 wherein saidintra-wire substructure of alternating elongated metal wires andelongated dielectric layers supports resonance enhanced tunnelingthrough said elongated metal wires, thereby enhancing transmission ofthe light of the polarization state orthogonal to said array ofcomposite wires.
 10. An imbedded wire grid polarizer according to claim6 wherein said intra-wire substructure of alternating elongated metalwires and elongated dielectric layers comprises at least one of saiddielectric layers.
 11. An imbedded wire grid polarizer according toclaim 6 wherein said incident light is within the range of approximately0.4 to 1.6 μm in the electro-magnetic spectrum.
 12. An imbedded wiregrid polarizer according to claim 6 wherein said first dielectricsubstrate and said second dielectric substrate are fabricated to form anintegrated polarizing device that is either a plate polarizer or a cubeprism polarizer.
 13. A modulation optical system for providing highcontrast modulation of an incident light beam, comprising: (a) a wiregrid polarization beamsplitter with a substrate having a surface; (i) anarray of parallel, elongated, composite wires disposed on said surface,wherein each of said composite wires are spaced apart at a grid periodless than a wavelength of said incident light; (ii) wherein each of saidcomposite wires comprises an intra-wire substructure of alternatingelongated metal wires and elongated dielectric layers; (iii) whereinsaid intra-wire substructure of alternating elongated metal wires andelongated dielectric layers comprises at least two of said elongatedmetal wires; and wherein said wire grid polarization beamsplittertransmits a first polarization state of said light beam and reflects asecond polarization state of said light beam, wherein said secondpolarization state is orthogonal to said first polarization state; (b) areflective spatial light modulator having a plurality of individualelements which alter said first polarization state of said transmittedlight beam to provide an image bearing beam; and (c) wherein said imagebearing beam reflects from said wire grid polarization beamsplitter. 14.A modulation optical system according claim 13 which includes apolarization analyzer which transmits said image bearing light beam andattenuates unwanted polarization components of said image bearing lightbeam.
 15. A modulation optical system according to claim 13 whichincludes a pre-polarizer for pre-polarizing said beam of light.
 16. Amodulation optical system according to claim 13 wherein said spatiallight modulator is a liquid crystal display device.
 17. A modulationoptical system according to claim 13 wherein said composite wires onsaid wire grid polarization beamsplitter face said reflective spatiallight modulator.
 18. A modulation optical system for providing highcontrast modulation of an incident light beam, comprising: (a) a wiregrid polarization beamsplitter with a substrate having a surface; (i) anarray of parallel, elongated, composite wires disposed on said surface,wherein each of said composite wires are spaced apart at a grid periodless than a wavelength of said incident light; (ii) wherein each of saidcomposite wires comprises an intra-wire substructure of alternatingelongated metal wires and elongated dielectric layers; (iii) whereinsaid intra-wire substructure of alternating elongated metal wires andelongated dielectric layers comprises at least two of said elongatedmetal wires; wherein said wire grid polarization beamsplitter reflects afirst polarization state of said light beam and transmits a secondpolarization state of said light beam, wherein said second polarizationstate is orthogonal to said first polarization state; (b) a reflectivespatial light modulator having a plurality of individual elements whichalter said first polarization state of said reflected light beam toprovide an image bearing beam; and (c) wherein said image bearing beamis transmitted through said wire grid polarization beamsplitter.
 19. Amodulation optical system according claim 18 which includes apolarization analyzer which transmits said image bearing light beam andattenuates unwanted polarization components of said image bearing lightbeam.
 20. A modulation optical system according to claim 18 whichincludes a pre-polarizer for pre-polarizing said beam of light.
 21. Amodulation optical system according to claim 18 wherein said spatiallight modulator is a liquid crystal display device.
 22. A modulationoptical system according to claim 18 wherein said composite wires onsaid wire grid polarization beamsplitter face said reflective spatiallight modulator.
 23. A modulation optical system for providing highcontrast modulation of an incident light beam, comprising: (a) a wiregrid polarization beamsplitter with a substrate having a surface; (i) anarray of parallel, elongated, composite wires disposed on said surface,wherein each of said composite wires are spaced apart at a grid periodless than a wavelength of said incident light; (ii) wherein each of saidcomposite wires comprises an intra-wire substructure of alternatingelongated metal wires and elongated dielectric layers; (iii) whereinsaid intra-wire substructure of alternating elongated metal wires andelongated dielectric layers comprises at least two of said elongatedmetal wires; wherein said wire grid polarization beamsplitter reflects afirst polarization state of said light beam and transmits a secondpolarization state of said light beam, wherein said second polarizationstate is orthogonal to said first polarization state; (b) a firstreflective spatial light modulator having a plurality of individualelements which alter said first polarization state of said reflectedlight beam to provide an image bearing beam, where said image bearingbeam is subsequently transmitted through said wire grid polarizationbeamsplitter; and (c) a second reflective spatial light modulator havinga plurality of individual elements which alter said second polarizationstate of said transmitted light beam to provide a second image bearingbeam, where said second image bearing beam is subsequently reflected offsaid wire grid polarization beamsplitter.
 24. A modulation opticalsystem according claim 23 which includes a polarization analyzer whichtransmits said first and second image bearing light beams and attenuatesunwanted polarization components of said image bearing light beams. 25.A modulation optical system according to claim 23 which includes apre-polarizer for pre-polarizing said beam of light.
 26. A modulationoptical system according to claim 23 wherein said first and secondspatial light modulators are liquid crystal display devices.
 27. Amodulation optical system for providing high contrast modulation of anincident light beam, comprising: (a) a wire grid polarizer with asubstrate having a surface; (i) an array of parallel, elongated,composite wires disposed on said surface, wherein each of said compositewires are spaced apart at a grid period less than a wavelength of saidincident light; (ii) wherein each of said composite wires comprises anintra-wire substructure of alternating elongated metal wires andelongated dielectric layers; (iii) wherein said intra-wire substructureof alternating elongated metal wires and elongated dielectric layerscomprises at least two of said elongated metal wires; wherein said wiregrid polarizer reflects a first polarization state of said light beamand transmits a second polarization state of said light beam, whereinsaid second polarization state is orthogonal to said first polarizationstate; (b) a spatial light modulator having a plurality of individualelements which alters said second polarization state of said light beamto provide an image bearing beam; (c) at least one additional polarizerto enhance the polarization contrast ratio of said image bearing beam;and wherein said spatial light modulator is located in an optical pathtraversed by said light beam such that it is between said wire gridpolarizer and said additional polarizer.
 28. A wire grid polarizer forpolarizing an incident light beam comprising: a substrate having asurface; an array of parallel, elongated, composite wires withintervening grooves disposed on said surface, wherein each of saidcomposite wires are spaced apart at a grid period; wherein each of saidcomposite wires comprises an intra-wire substructure of alternatingelongated metal wires and elongated dielectric layers; and wherein saidintra-wire substructure of alternating elongated metal wires andelongated dielectric layers comprises at least two of said elongatedmetal wires.
 29. A wire grid polarizer as in claim 28 wherein each ofsaid grooves are filled with a dielectric optical material.
 30. A wiregrid polarizer as in claim 29 wherein said dielectric optical materialis selected from a group comprised of an optically clear liquid,adhesive, and gel.
 31. A wire grid polarizer as in claim 29 wherein saiddielectric optical material has an index of refraction which is the sameas the index of refraction of said elongated dielectric layers.
 32. Awire grid polarizer as in claim 28 wherein said wire grid polarizer isoriented at an angle relative to said incident light beam.
 33. A wiregrid polarizer as in claim 28 wherein said intra-wire substructure ofalternating elongated metal wires and elongated dielectric layersprovide resonance enhanced tunneling through said elongated metal wires.34. A wire grid polarizer as in claim 33 wherein one of said dielectriclayers is adjacent to said surface of said substrate.
 35. A wire gridpolarizer as in claim 33 wherein one of said dielectric layers forms atop layer of said intra-wire substructure.
 36. A wire grid polarizer asin claim 33 wherein at least one of said dielectric layers has athickness different from each of said other dielectric layers.
 37. Awire grid polarizer as in claim 33 wherein at least one of saiddielectric layers has a different refractive index than said otherdielectric layers.
 38. A wire grid polarizer as in claim 28 wherein saidintra-wire substructure of alternating elongated metal wires andelongated dielectric layers comprises at least one of said dielectriclayers.
 39. A wire grid polarizer as in claim 28 wherein said intra-wiresubstructure of alternating elongated metal wires and elongateddielectric layers comprise a plurality of said dielectric layers.
 40. Awire grid polarizer as in claim 28 wherein one of said elongated metalwires is adjacent to said surface of said substrate.
 41. A wire gridpolarizer as in claim 28 wherein one of said elongated metal wires formsa top surface of said intra-wire substructure.
 42. A wire grid polarizeras in claim 28 wherein said elongated dielectric layers are thicker thansaid elongated metal wires.
 43. A wire grid polarizer as in claim 28wherein a thickness of said composite wire is between one-half to fivewavelengths of said incident light.
 44. A wire grid polarizer as inclaim 28 wherein said elongated metal wires have a thickness between 1-4skin depth thicknesses.
 45. A wire grid polarizer as in claim 28 whereinsaid elongated metal wires are selected from a group comprisingaluminum, gold, and silver.
 46. A wire grid polarizer as in claim 28wherein said elongated dielectric layers are selected from a groupcomprising MgF2, SiO2, and TiO2.
 47. A wire grid polarizer as in claim28 wherein a thickness of said composite wires varies across the surfaceof said substrate.